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Optik 124 (2013) 71– 73

Contents lists available at SciVerse ScienceDirect

Optik

jou rna l homepage: www.elsev ier .de / i j leo

daptive fibre nonlinearity precompensation based on optical performanceonitoring in coherent optical OFDM transmission systems

.X. Houa,b,∗, Q. Shia,b, Y.M. Luc, D. Liua,b

Research Institute of Highway Ministry of Transport, Beijing 100088, ChinaResearch Institute of Highway (RIOH) Transport Consultants Ltd, Beijing 100088, ChinaKey Laboratory of Trustworthy Distributed Computing and Service, Ministry of Education, Beijing University of Posts and Telecommunications, Beijing 100876, China

r t i c l e i n f o

rticle history:eceived 13 June 2011ccepted 12 November 2011

CIS:060.1660) Coherent communications060.2330) Fibre optics communications

a b s t r a c t

This paper forces on the monitoring and compensation of optical telecommunication channels. An adap-tive fibre nonlinearity precompensation (AFNP) scheme is proposed to solve the fibre nonlinearity incoherent optical orthogonal frequency division multiplexed networks (CO-OFDM). Optical performancemonitoring (OPM) at end-terminals is applied to channel identification in this paper. It is considered tobe high efficiency and a cost efficient technique in low-dynamic system.

© 2011 Elsevier GmbH. All rights reserved.

eywords:ibre nonlinearityoherent optical orthogonal frequencyivision multiplexed networksptical performance monitoring

recompensation

. Introduction

Coherent optical orthogonal frequency division multiplexedetworks (CO-OFDM) transmission technique has gained much

nterest in the field of optical fibre communication recently [1–6].t is a serious contender for the future optical fibre transmissionystems because it effectively removes the inter-symbol inter-erence from chromatic dispersion (CD) and polarization modeispersion (PMD) [2]. This system can apply flexible digital sig-al processing algorithm to compensate the chromatic dispersion,itiganting the urge for precise optical chromatic dispersion com-

ensation and reducing the cost of the system construction andperation. Its unique advantage has become prominent in high-peed and long-haul optical fibre transmissions. However, there isimitation on the performance in this system. The major reason foruch limitation originates from the nonlinearity impairment (suchs SPM (self-phase modulation), XPM (cross-phase modulation),WM (Four-wave-mixing) and so on) in the optical fibre. Due to

ompact distribution of sub-carriers, it is more severe than that inhe traditional system.

∗ Corresponding author. Tel.: +86 1082010916; fax: +86 1062370155.E-mail address: lancyhou@gmail.com (L.X. Hou).

030-4026/$ – see front matter © 2011 Elsevier GmbH. All rights reserved.oi:10.1016/j.ijleo.2011.11.045

Non-linear fibre effects should be mitiganted if the opticalpowers are kept low [7]. However, low powers require frequentreamplification to maintain a sufficient signal-to-noise ratio.Although, it is useful to try to mitigate fibre nonlinearity, the num-ber of optical amplifiers could also contribute impairment at thesame time. Fibre nonlinearity compensation was first proposedusing materials with a negative nonlinear coefficient [8], which isimpracticable. Recently, electronic dispersion compensation (EDC)is becoming an attractive technology for dispersion nonlinearitycompensation in optical links [5,6,9], where the system is mod-elled but with inverse parameters for dispersion and nonlinearcoefficients. However, detailed information of the dispersion mapand optical power levels along the path is required. Computationbecomes extensive for real-time implementation in high-dynamicsystem.

This paper presents a optical performance monitor (OPM) [10]to estimate nonlinearities compensation. In order to identify thelevel of non-linearity, we propose to apply a spectrum estimationof a representative linear system and an equivalent noise spectrumfor the non-linear distortion contribution. Simple signal process-ing at the transmitter can be used to implement compensation,

and this processing require asynchronous monitoring informationfrom OPM, so it tentatively does not work over a wide range ofhigh-dynamic systems because of OPM processing capacity. Thecomputation cost that used for dispersion compensation is the

72 L.X. Hou et al. / Optik 124 (2013) 71– 73

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hardware multipliers just after the inverse fast Fourier transform(FFT). These will adjust the phase of each time sample in proportionto its magnitude squared, as its magnitude squared modulates theoptical power, so it is implemented.

Fig. 1. Description of CO-OFD

ourier transforms in OFDM. To avoid overload computation inompensation implement, we select Freeway Private Optical Net-ork (FPON) as simulation object. In which, service is gathering

ype basically and detailed knowledge of the dispersion map cane obtained more easily.

. Precompensation algorithm

The OPM can monitor the optical power of each WDM channel,nd estimate the OSNR by interpolating the ASE level aside of theignal. In this scheme, the pilot tones technology is used to monitorarious optical parameters of signals. It can be extremely cost-ffective, because this can monitor without using the expensivee-multiplexing filters (such as tuneable optical filter and diffrac-ion grating) and it is also well suited for the use in dynamic opticaletworks. Fig. 1 depicts the investigated CO-OFDM transmissionystem. Two independent baseband signals modulate the orthog-nally polarized parts of the TX signal. Pilot tones are located atredetermined positions for optical modulator and they carry axed, in both magnitude and phase, symbol (known at the receiver).t the receiver, the OPM coherent detection is deployed. The trans-itter allows any optical amplitude and phase to be transmitted

long the link. The modulator produces a single-sideband opticalpectrum with a totally suppressed carrier. At the OFDM trans-itter, modulated symbols send by an inverse discrete Fourier

ransform (IDFT) before further processing and transmission. At theeceiver, the signal is sampled and given to DFT computation.

As the sampled spectra of the TX and RX signals represent byhe sub-carriers of OFDM symbols, the calculations can be directlyarried out on a number of OFDM symbols which are known to theeceiver. Obviously a certain number of preamble symbols have toe transmitted; alternatively one can think of analysis of data afterqualization and decision. Our proposed monitoring technique inPM is based on a system identification approach which describes

he optical transmission path.An ensemble of corresponding signal spectra X(f) and Y(f) which

re derived by Fourier transform leads to the linear transfer func-ion. The criterion of minimum power of the noise-like signalhould be used for the estimate of the linear transfer function com-utation. And the spectral power density of nonlinearity could alsoe estimated by another averaging process over the ensemble of

nput and output spectra. ˚nn(f) is the expectation who measureor non-linearity distortions.

nn(f ) = E

{∣∣∣∣∣Y(f ) − E{Y(f ) · X∗(f )}E{

∣∣X(f )∣∣2}

· X(f )

∣∣∣∣∣}

(1)

We integrate over the used frequency band, and work with sig-al powers rather than spectral power densities over the discreteub-carriers d:

N

S=

∑Dd=1˚nn(d)∑ ∣ ∣2

(2)

Dd=1

∣∣ E{Y(d)·X∗(d)}E{|X(d)|2}

· X(d)∣∣

In the following, an estimate of ˚nn(f) comprises two contri-utions: a noise-like signal due to non-linear effects of power Nnl

nsmission system with OPM.

along with actual additive noise (power: Nl). And we can extractthe wanted ratios Nnl/S and Nl/S using further computation.

The OFDM precompensation method proposed here uses thelow walk-off to an advantage because the transmission path haslow Chromatic Dispersion which can be approximated compensa-tion when calculating the effects of fibre nonlinearity [11]. Becausefibre dispersion mitigates the effect of nonlinearity because ofsome walk-off, the effective length for precompensation will be lessthan its dispersionless value [12]. Actually a single rectification isrequired for the entire nonlinear computation.

To represent fibre nonlinearity, the following revised calculationapplies a phase advance, �(t) in proportion to the instantaneousoptical power P(t), that is input to the first fibre span [13]

�(t) = P(t) · sLeff · 2�n2

(�0Aeff ) − �0(3)

where s is the number of fibre spans, Leff is an effective length ofeach fibre span for nonlinearity compensation, n2 is a nonlinear-ity coefficient, �0 is the wavelength of the carrier, and Aeff is theeffective core area of the fibre, ϕ0 is the phase adjustment of CD.

The phase adjustment of CD can be obtained by solving nonlin-ear Schrdinger equations. Thus, now equation reads:

�0 = ˚0 + c�

f 2LD

Dtf2 (4)

where fLD is the centre frequency of laser, Dt denotes dispersionparameters and ˚0 is the phase shift according to centre frequency.

This offset for compensation could be estimated at the receiver,where the instantaneous optical power is proportional to thesquare of the sum of the fields of the subcarriers, as would bedetected by a photodiode. More conveniently, it can be sent totransmitter and applied in the electrical domain using a bank of

Fig. 2. The Freeway Private Optical Network (FPON) topology.

L.X. Hou et al. / Optik 12

Fig. 3. BER versus SNR in single link.

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Fig. 4. Blocking probability under various networks load.

. Simulation results

In this section, we evaluate the AFNP using the FPON topology.n order to achieve a realistic network load, lightpath provisioningequests are dynamically generated according to a Poison processnd uniformly distributed among the source-destination pairs. Theata rate is 10 Gb/s and the block length is 1024 bits, giving D = 128ub-carriers of OFDM symbols in an optical bandwidth of 5 GHzith 4-quadrature amplitude modulation (QAM). Each link com-rises number of uncompensated 80-km spans. The fibre has a lossf 0.2 dB/km, a nonlinearity coefficient, n2, of 2.6 × 10−20 m2/W, andn effective cross section of 80 �m2. The optical amplifiers compen-ate for the 16-dB fibre loss in each span, and have a noise figureNF) of 6 dB. The output power of each amplifier is controlled to sethe input power to each 80-km fibre span. The coherent receiversed a 10-mW local oscillator laser and is noiseless (Fig. 2).

Fig. 3 plots BER versus SNR for both systems in the 4000-kmingle link (from Urumchi to Beijing) and back to back system. For

BER of 10-3 (which can be improved by Forward-Error Correctionoding), the AFND system requires a 3 dB better SNR. This advantage

f OFDM over AFDN reduces to zero for lower BERs, but only ifhe AFDN system’s threshold is optimized to take advantage of theow variance of the zero-bits for high extinction ratios. If the AFDNhreshold is placed midway between the 1 and 0 levels (as it would

[[

4 (2013) 71– 73 73

for poor extinction ratios), OFDM has a 1.6-dB advantage over AFDNat all OSNRs. Plots of the variance of the symbols for OSNRs up to20 dB suggest that a BER floor does not occur for OFDM.

In Fig. 4, we compare the performance among the three schemesunder various traffic of services using the FPON. The blocking proba-bility (BP) denotes the effectivity for each scheme. The figure showsthat the BP without AFNP are higer than those two schemes unlessunder heavy traffic load (700 Erlang). Actually, on this occasion,processing capacity is the biggest bottleneck in AFNP. The systemwith AFNP also performs similar to back to back under low trafficload, and even under middle, it still remains 99% restoration prob-ability in 600 Erlang. The service of FPON in normal use actually isthe just one who has low-dynamic and high stability feature.

4. Conclusions

In this paper, we have shown that the CO-OFDM network isextended for the adaptive fibre nonlinearity precompensation todeal with optical impairments from nonlinearity. The AFNP extendsan OPM module to monitor the optical power and identify systemcompensation. Simulation shows that the AFNP achieves betterperformance under low-dynamic. Therefore, the AFNP is a suit-able nonlinearity precompensation system in specified CO-OFDMnetworks.

Acknowledgement

This research was supported by The National Basic Research(973) Program of China (No. 2011CB302702), The National 863Program (No. 2011AA01A205).

References

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